I'm old enough to have been prescribed the liquid-filled fentanyl patches.
After 72 hours almost 50% of the liquid remained. So I would squeeze it onto foil and use a marker pen to outline where on the foil it was. Placed in an oven at 40C for 1 hour and it was invisible - only the marker pen showed where it was.
Carefully smoked invisible solid (fentanyl citrate MP: 149-151°C) and it worked. Well, the bland, non-euphoric high lasted the whole 20 minutes... then I redosed.
But although glad to have tried - it's going to be a f**king disaster in the US and Canada by which I mean 'you haven't seen anything yet.
What annoys me is that George Marquardt began making α-methyl fentanyl, an analogue with a duration of 4 hours. It was initially used as an active cut for heroin but eventually the HUGE profits meant people were selling it well cut (usually), it was identified and he was caught.
Mexican (and US and Canadian) manufacturers could make it - better they could make α-methyl thiofentanyl (the intermediates are not controlled AND it's more potent). Also - if fentanyl is worth $4150/Kg (pure & in vast bulk but wholesales for $80000/Kg) then something with the duration of H is going to be worth more - heck, sell it as H.
But customers will accept fentanyl and Grisham's law does the rest.
For my money, levorphanol & oxymorphone derivatives represent a much better product that could be sold for considerably more. After all, didn't 40mg oxymorphone tablets sell for $80 each? So a Kg would retail for $1 million? But kids don't even remember oxymorphone. If you swap oxyM for, say N-phenylethyl noroxymorphone and it's a magnitude stronger.... so 4mg would feel like 40mg of oxyM. So add a 0 to the end of the profit.